540 



Geological Society. 



and Orclovician rocks. In the southern part of the peninsula the- 

 structure is relatively simple, and the succession very plainly dis- 

 played in numerous cliff-sections ; Cambrian rocks very similar in 

 character to those of Merionethshire form most of the coast, but 

 the interior is mainly occupied by Arenig beds, which rest with a 

 marked unconformity on every local member of the Cambrian in turn. 

 The latter have fortunately escaped cleavage, and some mudstones. 

 in the midst of the series have yielded a number of fossils belong- 

 ing to the zone of Paradoxides liicJcsi. The P.-davidis Zone 

 appears to be absent, as the result of a slight unconformity. 



This southern area, of relatively simple structure, is separated by 

 an overthrust of considerable magnitude from a more northern 

 area in which members of the Tremadoc, Arenig, and Llandeilo 

 Series have been recognized, but in which the rocks are highly 

 crushed, faulted, and" disturbed, and the relations between the beds- 

 are far from clear. 



The general succession is : — 



South of the Thrust. 



Llanengan Mudstones. 



Tudwal Sandstones and 



Grits. 

 (Zone of Didymograptus 

 extensus.) 



Great unconformity. 



1 



Llandeilo. 



Llanvirn. 



Arenig. 



Tremadoc. 



Dolgelly. 

 Ffestiniog. 



North of the Thrust. 



Hen-dy-Capel Mudstones. 

 (Zone of Nemagraptus 

 gracilis.) 



Benar Beds. 

 (Zone of Didymograptus 



extensus ?) 

 Probable unconformity. 



Abersoch Beds. 



Ffestiniog- Beds. 



(Only in East St. Tudwal's Island.) 

 Maentwrog Beds. Maentwrog. 



Probable unconformity. 

 Nant-pig Mudstones. Menevian (of Merionethshire). 



Caered Mudstones and Flags. ~} 



Cilan Grits. I Harlech Grits (of Merionethshire) 



Mulfran or Manganese Beds. [ (in part). 

 Hell's Mouth Grits. J 



Base not seen. 



Pisolitic iron-ore is well developed in the district, and occurs 

 chiefly in the Llandeilo Beds along the line of the overthrust ; it is 

 regarded as of sedimentary origin. The glacial geology is only 

 briefly dealt with ; but evidence is presented to show that,, during 

 the last phase of glaciation, the ice was moving across the peninsula 

 in a westerly direction out of Cardigan Bay. 



